To get the Terminal Velocity achievement in Portal, you need to have both orange and blue portals and be in Chapter 15. You also need to continue to fall for a total of 5 minutes. There aren’t any places in the Aperture Science Enrichment Center where you can fall far enough, but your Portal Gun allows you to fall indefinitely if you wish. To do this, you will need to have reached Test Chamber 11, where you gain the ability to create both blue portals and orange portals.